64. BULLETIN OF THE 
servations appear to me to afford considerable evidence in favor of the 
view that the hexagonal arrangement is phylogenetically more primitive 
than the tetragonal. 
Granting this conclusion, a number of otherwise exceptional observa- 
tions can be explained. Thus, as long ago as 1840, Will (40, p. 7) 
called attention to the fact that in Astacus, where the ommatidia are 
normally arranged upon the tetragonal plan, facets near the edge of the 
retina are often irregularly hexagonal. The edge of the retina is well 
known to be the last part produced, and therefore it is probably the 
part least differentiated. Admitting the hexagonal arrangement to be 
a primitive one, it is only natural to expect that, if it persists at all, 
it will persist in the less modified portion of the retina. Hexagonal. 
facets also occur on the periphery of the retina in Homarus, and are to 
be explained, I believe, in the same way. 
On the assumption that the hexagonal plan is primitive, the occur- 
rence of a few genera with ommatidia hexagonally arranged, in a group 
in which the tetragonal ‘arrangement is the rule, can also be explained. 
In Typton, for instance, the hexagonal plan obtains, although in almost all 
Crustaceans closely related to it the tetragonal system prevails. This 
condition may be explained, however, by the fact that the eyes in Typton 
show evident signs of degeneracy, due in all probability to the parasitic 
habits of the Crustacean. If the hexagonal arrangement represents an 
early ontogenetic phase in the development of Decapods related to Typ- 
ton, it would be natural to expect that in Typton itself, where the normal 
development of the eyes is interrupted by parasitism, this arrangement 
would persist permanently. 
In Galathea, as I have already mentioned in a note on page 63, the 
ommatidia according to Will are arranged tetragonally ; according to 
Patten, hexagonally. At first sight these observations might appear 
to be irreconcilable, but such is not necessarily the case. So far as I 
have been able to ascertain, Patten does not mention the name of the 
species which he studied. Possibly he may have examined some other 
than G. strigosa, the one from which Will’s figures were drawn. In 
such an event, a difference in the arrangement of the ommatidia may 
have been characteristic of the two species, although, if both possessed 
well developed eyes, this difference would be somewhat anomalous. If 
this is not the true explanation, it is still possible that the specimens 
studied by Patten were somewhat immature, in which case the hexagonal 
arrangement might very naturally be present. From what has been said, 
I think it must be evident that the apparent contradiction in Will’s and 
